Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens composed of six optical elements which is built in an image pickup device mounted in an increasingly compact and low-profile smartphone or mobile phone, PDA (Personal Digital Assistant), game console, information terminal such as a PC, or home appliance with a camera function.
In the present invention, whether or not an optical element is a lens is decided depending on whether or not it has refractive power on an optical axis. An optical element having refractive power on the optical axis is called a lens. An optical element which does not have a lens function does not change the overall focal length and can contribute to improvement of aberrations in the peripheral area. In terms of lens surface shape, a convex surface or a concave surface means that the paraxial portion of the surface (portion near the optical axis) is convex or concave. A “pole-change point” on an aspheric surface means a point on the aspheric surface at which a tangential plane intersects the optical axis perpendicularly.
Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera have been introduced into the market. For example, a user who is away from home can see in real time what is going on at home, through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smartphone. It is thought that products which enhance consumer convenience by adding a camera function to an information terminal or home appliance will be increasingly developed in the future. The camera mounted in such product is expected not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view. In particular, the imaging lens to be built in a mobile terminal is strongly expected to be low-profile enough to be applicable to a low-profile product.
However, in order to provide a low-profile imaging lens with a wide field of view and high brightness as described above, the following problem has to be addressed: it is difficult to correct aberrations in the peripheral area of the image and deliver high imaging performance throughout the image.
Conventionally, for example, the imaging lenses described in JP-A-2010-026434 (Patent Document 1) and JP-A-2011-085733 (Patent Document 2) are known as compact high-resolution imaging lenses.
Patent Document 1 discloses a compact imaging lens composed of five constituent lenses in which a first positive lens, a second positive lens, a third negative lens, a fourth positive lens and a fifth negative lens are arranged in order from an object side. The imaging lens offers brightness with an F-value of about 2 and corrects various aberrations properly.
Patent Document 2 discloses an imaging lens which includes, in order from an object side, a first lens group including a first lens having a convex surface on the object side, a second lens group including a second lens having a concave surface on an image side, a third lens group including a third meniscus lens having a concave surface on the object side, a fourth lens group including a fourth meniscus lens having a concave surface on the object side, and a fifth lens group including a fifth meniscus lens having an aspheric surface with an inflection point on the object side. This configuration is intended to provide a compact imaging lens system which offers high resolution.
The imaging lens described in Patent Document 1, composed of five constituent lenses, corrects various aberrations properly and offers high brightness with an F-value of about 2.0 to about 2.5; however, its total track length is longer than the diagonal length of the effective imaging plane of the image sensor, which is disadvantageous in making the imaging lens low-profile. Furthermore, if this lens configuration is designed to offer a wide field of view, it will be difficult to correct aberrations in the peripheral area of the image.
The imaging lens described in Patent Document 2 is a relatively low-profile lens system which correct aberrations properly. However, in order for this configuration to offer brightness with an F-value of 2.8 or less and a field of view of 65 degrees or more, it is necessary to address the problem with difficulty in correction of aberrations in the peripheral area of the image.
As mentioned above, conventionally, it is difficult to provide a low-profile high-resolution imaging lens which offers a wide field of view and high brightness.